1. Field of the Invention
The present invention relates to an alkali-free glass substrate, more particularly to an alkali-free glass substrate useful as a transparent glass substrate for displays and others.
2. Description of Related Art
Conventionally, an alkali-free glass has been used as a transparent glass for liquid crystal displays. The alkali-free glass particularly for display application is required to be free of bubbles that result in display defects, as well as having satisfactory heat resistance and chemical durability.
For such an alkali-free glass, various types of glasses have been proposed. For example, Japanese Patent Kokai No. Hei 6-263473 and Japanese Patent Kohyo No. 2001-500098 disclose alkali-free aluminosilicate glasses.
If a bubble-free glass is to be obtained, a gas produced during a batch decomposition must be expelled from a glass melt by a fining gas. In addition, the remaining minute bubbles must be removed during homogenization melting by the reproduced fining gas which renders them larger in sizes and thereby causes them to float in the melt.
For the alkali-free glass, particularly for use as a glass substrate for liquid crystal displays, a glass melt has a high viscosity and its melting is carried out at higher temperatures compared to alkali-containing glasses.
For this type of alkali-free glass, a batch decomposition occurs generally at 1,300-1,500° C. Accordingly, fining and homogenization are carried out at 1,500° C. or higher temperatures. Under such conditions, As2O3 has been widely used as a fining agent which can release a fining gas over a wide temperature range (approximate range of 1,300-1,700° C.).
However, As2O3 is very toxic in nature and possibly causes environmental pollution during glass manufacturing processes or waste glass treatments. For this reason, its use is being increasingly limited.